The following discussion is intended to facilitate the understanding of the invention, but is not intended nor admitted to be prior art to the invention.
A. Osteoporosis
Osteoporosis is a disabling disease characterized by the loss of bone mass and microarchitectural deterioration of skeletal structure leading to compromised bone strength, which predisposes a patient to increased risk of fragility fractures. Osteoporosis affects more than 75 million people in Europe, Japan and the United States, and causes more than 2.3 million fractures in Europe and the United States alone. In the United States, osteoporosis affects at least 25% of all post-menopausal white women, and the proportion rises to 70% in women older than 80 years. One in three women older than 50 years will have an osteoporotic fracture that causes a considerable social and financial burden on society. The disease is not limited to women; older men also can be affected. By 2050, the worldwide incidence of hip fracture in men is projected to increase by 310% and 240% in women. The combined lifetime risk for hip, forearm, and vertebral fractures presenting clinically is around 40%, equivalent to the risk for cardiovascular disease. Osteoporotic fractures therefore cause substantial mortality, morbidity, and economic cost. With an ageing population, the number of osteoporotic fractures and their costs will at least double in the next 50 years unless effective preventive strategies are developed. (See, e.g., Atik et al., Clin Orthop Relat Res (2006) 443:19-24; Raisz, J Clin Invest (2005) 115:3318-3325; and World Health Organization Technical Report Series 921 (2003), Prevention and Management of Osteoporosis.)
B. Glucose-Dependent Insulinotropic Polypeptide (GIP)
Glucose-dependent insulinotropic polypeptide (GIP, also known as gastric inhibitory polypeptide) is a peptide incretin hormone of 42 amino acids that is released from duodenal endocrine K cells after meal ingestion. The amount of GIP released is largely dependent on the amount of glucose consumed. GIP has been shown to stimulate glucose-dependent insulin secretion in pancreatic beta cells. GIP mediates its actions through a specific G protein-coupled receptor, namely GIPR.
As GIP contains an alanine at position 2, it is an excellent substrate for dipeptidyl peptidase-4 (DPP-IV), an enzyme regulating the degradation of GIP. Full-length GIP(1-42) is rapidly converted to bioinactive GIP(3-42) within minutes of secretion from the gut K cell. Inhibition of DPP-IV has been shown to augment GIP bioactivity. (See, e.g., Drucker, Cell Metab (2006) 3:153-165; McIntosh et al., Regul Pept (2005) 128:159-165; Deacon, Regul Pept (2005) 128:117-124; and Ahren et al., Endocrinology (2005) 146:2055-2059.) Analysis of full length bioactive GIP, for example in blood, can be carried out using N-terminal-specific assays (see, e.g., Deacon et al, J Clin Endocrinol Metab (2000) 85:3575-3581).
Recently, GIP has been shown to promote bone formation. GIP has been shown to activate osteoblastic receptors, resulting in increases in collagen type I synthesis and alkaline phosphatase activity, both associated with bone formation. GIP has been shown to inhibit osteoclast activity and differentiation in vitro. GIP administration has been shown to prevent the bone loss due to ovariectomy. GIP receptor (GIPR) knockout mice evidence a decreased bone size, lower bone mass, altered bone microarchitecture and biochemical properties, and altered parameters for bone turnover, especially in bone formation. (See, e.g., Zhong et al, Am J Physiol Endocrinol Metab (2007) 292:E543-E548; Bollag et al., Endocrinology (2000) 141:1228-1235; Bollag et al., Mol Cell Endocrinol (2001) 177:35-41; Xie et al., Bone (2005) 37:759-769; and Tsukiyama et al., Mol Endocrinol (2006) 20:1644-1651.)
The usefulness of GIP for maintaining or increasing bone density or formation has been acknowledged by the United State Trademark and Patent Office by issuance of U.S. Pat. No. 6,410,508 for the treatment of reduced bone mineralization by administration of GIP peptide. However, current GIP peptide agonists suffer from a lack of oral bioavailability, negatively impacting patient compliance. An attractive alternative approach is to develop an orally active composition for increasing an endogenous level of GIP activity.
C. GPR119
GPR119 is a G protein-coupled receptor (GPR119; e.g., human GPR119, GenBank® Accession No. AAP72125 and alleles thereof; e.g., mouse GPR119, GenBank® Accession No. AY288423 and alleles thereof). GPR119 activation as by an agonist leads to elevation of the level of intracellular cAMP, consistent with GPR119 being coupled to Gs. In the patent literature, GPR119 has been referred to as RUP3 (e.g., WO 00/31258); GPR119 has also been referred to as Glucose-Dependent Insulinotropic Receptor (GDIR).
D. G Protein-Coupled Receptors
Although a number of receptor classes exist in humans, by far the most abundant and therapeutically relevant is represented by the G protein-coupled receptor (GPCR) class. It is estimated that there are some 30,000-40,000 genes within the human genome, and of these, approximately 2% are estimated to code for GPCRs.
GPCRs represent an important area for the development of pharmaceutical products. Drugs active at GPCRs have therapeutic benefit across a broad spectrum of human diseases as diverse as pain, cognitive dysfunction, hypertension, peptic ulcers, rhinitis, and asthma. Of the approximately 500 clinically marketed drugs, greater than 30% are modulators of GPCR function. These drugs exert their activity at approximately 30 well-characterized GPCRs. (See, e.g., Wise et al, Annu Rev Pharmacol Toxicol (2004) 44:43-66.)
GPCRs share a common structural motif, having seven sequences of between 22 to 24 hydrophobic amino acids that form seven alpha helices, each of which spans the membrane (each span is identified by number, i.e., transmembrane-1 (TM-1), transmembrane-2 (TM-2), etc.). The transmembrane helices are joined by strands of amino acids between transmembrane-2 and transmembrane-3, transmembrane-4 and transmembrane-5, and transmembrane-6 and transmembrane-7 on the exterior, or “extracellular” side, of the cell membrane (these are referred to as “extracellular” regions 1, 2 and 3 (EC-1, EC-2 and EC-3), respectively). The transmembrane helices are also joined by strands of amino acids between transmembrane-1 and transmembrane-2, transmembrane-3 and transmembrane-4, and transmembrane-5 and transmembrane-6 on the interior, or “intracellular” side, of the cell membrane (these are referred to as “intracellular” regions 1, 2 and 3 (IC-1, IC-2 and IC-3), respectively). The “carboxy” (“C”) terminus of the receptor lies in the intracellular space within the cell, and the “amino” (“N”) terminus of the receptor lies in the extracellular space outside of the cell.
Generally, when a ligand binds with the receptor (often referred to as “activation” of the receptor), there is a change in the conformation of the receptor that facilitates coupling between the intracellular region and an intracellular “G-protein.” It has been reported that GPCRs are “promiscuous” with respect to G proteins, i.e., that a GPCR can interact with more than one G protein. See, Kenakin, Life Sciences (1988) 43:1095-1101. Although other G proteins exist, currently, Gq, Gs, Gi, Gz and Go are G proteins that have been identified. Ligand-activated GPCR coupling with the G-protein initiates a signaling cascade process (referred to as “signal transduction”). Under normal conditions, signal transduction ultimately results in cellular activation or cellular inhibition. Although not wishing to be bound to theory, it is thought that the IC-3 loop as well as the carboxy terminus of the receptor interact with the G protein.
There are also promiscuous G proteins, which appear to couple several classes of GPCRs to the phospholipase C pathway, such as G15 or G16 (Offermanns & Simon, J Biol Chem (1995) 270:15175-80), or chimeric G proteins designed to couple a large number of different GPCRs to the same pathway, e.g. phospholipase C (Milligan & Rees, Trends in Pharmaceutical Sciences (1999) 20:118-24).
Under physiological conditions, GPCRs exist in the cell membrane in equilibrium between two different conformations: an “inactive” state and an “active” state. A receptor in an inactive state is unable to link to the intracellular signaling transduction pathway to initiate signal transduction leading to a biological response. Changing the receptor conformation to the active state allows linkage to the transduction pathway (via the G-protein) and produces a biological response.
A receptor may be stabilized in an active state by a ligand or a compound such as a drug. Recent discoveries, including but not exclusively limited to modifications to the amino acid sequence of the receptor, provide means other than ligands or drugs to promote and stabilize the receptor in the active state conformation. These means effectively stabilize the receptor in an active state by simulating the effect of a ligand binding to the receptor. Stabilization by such ligand-independent means is termed “constitutive receptor activation.”